This invention relates generally to firing circuits for thyristor power conversion systems and more particularly to an improved means for controlling the operation of an alternating current (AC) motor drive system including an electrical load such as an AC synchronous motor supplied from a polyphase alternating current source.
Many circuits and systems are known for controlling the conductivity of controlled rectifiers utilized in various types of converters for supplying electrical power to a load, such as an AC motor, from a polyphase AC source. The type of rectifier used will, of course, control to some degree the type of control utilized but by far the most common controlled rectifier used today is a thyristor of the silicon controlled rectifier type. The thyristor becomes conductive with the simultaneous application of a forward bias voltage and a signal applied to its gate electrode and thereafter remains conductive until the anode current falls below the value required to hold the thyristor in the conductive state.
With respect to a load commutated inverter for a three phase (3.0.) AC motor drive, the line to line voltage is a function of the motor's back electromotive force (emf) which acts to commutate the inverter thyristors once the synchronous motor exceeds a predetermined low value of speed. Control of the inverter supplying the synchronous motor, furthermore, is normally based upon the desire to fire the thyristors as late as possible. To "fire as late as possible" is to render the thyristors conductive at the commutation limit points, i.e. at a power factor angle just sufficiently leading to provide the volt-seconds necessary to safely commutate the current transfer from one thyristor to the other in a manner well known to those skilled in the art.
It should also be pointed out that whereas motor control systems employing thyristors have been implemented using analog control techniques, a typical example being U.S. Pat. No. 4,230,979, entitled "Controlled Current Inverter And Motor Control System", Paul M. Espalage, et al., Oct. 28, 1980, attention has also been directed to digital type of control techniques, examples of which are disclosed for example in: U.S. Pat. No. 3,601,674, "Control System For Firing SCR's In Power Conversion Apparatus", John A. Joslyn, et al., issued on Aug. 24, 1971; U.S. Pat. No. 4,263,557, "Power Converter Control", Willard B. Jarvinen, issued on Apr. 21, 1981; and U.S. Pat. No. 4,276,505, "Microcomputer--Based Control Apparatus For A Load--Commutated Inverter Synchronous Machine Drive System", Bimal K. Bose, issued on June 30, 1981. The teachings of these patents are specifically intended to be incorporated herein by reference for enabling one skilled in the art to understand the present invention without the disclosure of extraneous and irrelevant material.
It should also be pointed out that one characteristic of a synchronous AC machine, and more particularly a synchronous motor, is that its terminal voltage at any given value of field excitation is very sensitive to the load current when the motor is not tied to a nominal fixed voltage bus. This fact is of particular significance when the motor is driven by a variable current source such as an inverter, for example, as disclosed in the aforementioned Bose patent. Under these circumstances, particularly when operating near rated load conditions, a small drop in load current can result in an appreciable rise in terminal voltage, i.e. the voltage appearing across the motor from the inverter. This increase in terminal voltage may be unacceptably high in certain instances as seen by the inverter and/or the motor which in certain instances can have catastrophic effects.